This invention relates to optoelectronic devices and more particularly to a light emitting device fabricated from an indirect bandgap semiconductor material and to a method of generating light in an indirect bandgap semiconductor material.
A multi-terminal light emitting device fabricated form an indirect bandgap semiconductor material, such as silicon, is disclosed in U.S. Pat. No. 6,111,271. The multi-terminal device comprises a first pn-junction and a second pn-junction. Terminals are provided, in use, to reverse bias the first junction into avalanche or field emission mode to generate light and to forward bias the second junction, to inject “cool” carriers into the avalanching or field emission region, thereby to cause radiative recombination of the “cool” carriers and “hot” carriers energized by the avalanching or field emission and to increase the light generation efficiency of the device.
However, in devices fabricated in accordance with a standard CMOS process, it has been found that due to the lateral layout of such devices, a limited percentage of the injected carriers reaches the region of the reverse biased junction that is in avalanche or field emission breakdown. A majority of injected carriers are injected via a bottom wall of the second junction vertically into the bulk material to recombine in the bulk or laterally in other directions via sidewalls of the second junction, so that only a fraction of the injected carriers reach the region of the reverse biased junction that is in avalanche or field emission breakdown.
Another problem is that buried oxide isolation regions in the bulk material, a typical result of local oxidation or shallow trench isolation techniques, which regions are used to reduce the sidewall effect of the junctions, may shield injected carriers from the avalanche or field emission region, thereby further reducing the number of injected carriers that reach the avalanche or field emission region.